In LED driver schemes in which the LED current, and thereby the grid current, are subject to linear control, and in which, due to their power consumption, it is necessary to ensure that the current drawn from the grid is largely sinusoidal, a setpoint value for the current controller has until now been derived by means of a voltage divider connected to the input voltage with a first and a second ohmic resistor. Since this input voltage is sinusoidal, both the setpoint value and, with an appropriate control concept, also the actual value of the grid current, are therefore also sinusoidal.
There are LED arrangements in which a grid current can only flow when the grid voltage is greater than the forward bias voltage of at least a portion of the LEDs used. These are LED arrangements in which, while particular LEDs of the overall arrangement can indeed be bridged by switches, a specific number of LEDs, in the present case what is known as the first cascade of LEDs are however not bridged, so saving a switch. Such arrangements are subject to the problem that the voltage tap at the voltage divider referred to above also outputs a non-negligible setpoint value during a period of time surrounding the zero transition of the grid, but a grid current corresponding to this setpoint value cannot flow. Through the use of a non-bridgeable cascade of LEDs it is possible to save one switch in the practical implementation. This cascade of LEDs is, accordingly, always in operation, provided the voltage provided at the output of the rectifier is larger than the forward bias voltage of the LEDs of this first cascade. This furthermore entails the advantage that, as long as the required forward bias voltage has not yet been reached, no current, which would merely generate heat loss there, can flow through the linear controller.
When conventional control apparatus is used for the current control this has the result that, starting at the moment at which the instantaneous value of the grid voltage drops below the forward bias voltage of the non-bridgeable portion of the LEDs, the current controller goes into saturation. When, subsequently, with increasing grid voltage it again rises above the forward bias voltage of the non-bridgeable portion of the LEDs, the current controller needs a settling time, during which the grid current is greater than the desired value corresponding to the setpoint value (control deviation). This overshoot in the grid current has a negative effect on the behavior of the overall arrangement in respect of grid current harmonics and of radio interference.
It would be conceivable, in order to solve this problem, to change the time-characteristic of the current controller in such a way that the gaps in the current are “masked out”. This, however, would entail the disadvantage that the overall speed of the current control could become too small.